Colour images may be obtained by exposing photographic material to light reflected or transmitted by a natural scene and subsequent processing. It may be desirable to reproduce such colour images in large quantities, using methods of electronic image processing. To achieve such a reproduction, the colour image may be scanned by an electronic colour scanner, such as the Agfa SelectScan (SelectScan is a trade mark of Agfa-Gevaert N.V.). Such a scanner divides the image into adjacent square or rectangular picture elements or pixels, and usually assigns to each pixel three colour values, identified as the red, green and blue pixel value (RGB) respectively. By characterisation of the electronic scanner for colour images, each triplet of colour values RGB may be converted to a point within a device independent colour space, such as CIE-XYZ, CIE-L* u* v* , etc. After that transformation, each such point may be transformed to a device dependent triplet, 4-tuple, etc. of required stimulus values for an output device, such as a thermal dye colour printer, an electrophotographic colour printer, or an image setter for generating three, four or more separation films or printing plates, to be used in a colour printing process, such as lithography, flexography, offset, etc. Recently High Fidelity or HiFi colour reproduction systems have gained more attention. As such, images may be presented in a corresponding HiFi colour space, such as CMYKOG, meaning cyan, magenta, yellow, black, orange and green, where six inks having these colors are used in the printing process. An image as observed by the acquisition device, or any electronic image, given by its RGB colour values, device independent colour values or output device dependent stimulus values may be regarded as an original image.
It is possible that colours on the original scene or in the original image need to be reproduced differently than as they were captured. This may be due to inadequate illumination conditions, improper settings of the image acquisition devices in the chain, or colour changes which must be deliberately imposed on the original image. In such cases, specific colours need to be changed, while affecting other colours less dramatically. In a page layout program Adobe Photoshop version 3.0 (Adobe Photoshop is a trademark of Adobe Systems Incorporated which may be registered in certain jurisdictions) colour adjustments are available under a "variations feature", that allows easy adjustment of image colour and brightness by previewing and choosing from a range of colour options; adjustments for brightness, contrast and midtones (gamma); controls for selectively adjusting hue, saturation and brightness; adjustable tonal curves and control points on curves; replacement of colour for correcting the colour of a selected area by adjusting its hue, saturation and brightness values; selective colour correction for adjusting the ink values of individual colour channels or plates by entering absolute or relative values; independent colour balance adjustment for shadows, highlights and midtones. In this version, it is possible to change the vertices of the RGB cube, i.e. red, green, blue, cyan, magenta, yellow, black and white, by means of sliders. It is also possible to change a neutral or grey point in the colour space. The sliders are going from -100% to +100%, and can be set in an absolute or relative mode. In the relative mode, the percentage is relative to the amount of CMYK of the colour changed. In the absolute mode, the change is the amount of extra or less ink in percentage. Whenever a slider has been moved, the image is updated interactively.
The method described above has some shortcomings. First of all, it is not possible to indicate changes in an arbitrary chosen point. Furthermore, it is not possible to get control to what extent other colours are affected by the required change to the selected colour.
The Color Companion plug-in for Photoshop, distributed by Van Ginneken & Mostaard, offers a number of fixed transformations, called flavours. These fixed transformations are stored on disk. The user may interactively select a number of these transformations, and apply one after the other to obtain a most desired colour correction. The chain of transformations may be combined and treated as a new transformation, The disadvantage of fixed transformations is that the user cannot specify transformations in an absolute way, i.e. by specifying which selected colour should be changed. By applying the transformations one after the other, the latter transformations may be influenced by the previous. The elimination of one transformation in the beginning of the chain may influence other transformations considerably.
DE 43 43 362 A1 discloses a method for selective colour correction, by selecting a plurality of colours, defining for each colour a colour change, computing weighting values for each colour in a local colour gamut, according to the presence of a selected colour and a convolution matrix, or computing for each point within the local colour gamut a new colour change, by convolving the original colour changes with such a convolution matrix. A disadvantage with this method is that either the weighting values are not guaranteed to have value one at the selected colour, or that the new colour changes are not exactly the required colour changes in the selected colours, which may be derived from FIG. 10e and 12c respectively.
GB 2 117 902 A discloses a method for selective colour correction, in which a "sample", comprising a large amount of colours, is selected, a required colour change is given for that sample, and a weighting function is defined for computing colour modifications. According to this method it depends on the distribution of colours within the sample having the same lightness, which colour value (1.x.sub.0.y.sub.0) will get the required colour change. This way, the interactive operator has no real control over a selected colour which must get a required colour change, since x.sub.0 and y.sub.0 are merely mean values for different x and y values corresponding to colour points in the original image having the same lightness.
EP 0 441 558 A1 also discloses a method for selective colour correction, in which a colour is selected and target colours are defined. According to this method it is not guaranteed that each selected colour will be corrected according to the target colour.
EP 0 566 914 A1 discloses a method for selective colour correction, by definition of a colour to be corrected, a target colour and an effective range. Although according to this method it may be guaranteed that selected colours are corrected to target colours, this method is silent about selective colour correction according to different target colours in which it is guaranteed that all selected colours are corrected according to the required target colour.